The present invention relates to solid waste disposal, and, more particularly, to apparatuses, systems, and methods for transforming solid waste into useful products, including a reusable, treatable, or readily degradable material.
Solid waste disposal can generally be defined as the disposal of normally solid or semi-solid materials, resulting from human and animal activities, which are useless, unwanted, or hazardous. “Solid waste” generally comprises “garbage,” including decomposable wastes from food; “rubbish” including combustible decomposable wastes, such as paper, wood, and cloth, or non-combustible decomposable wastes, such as metal, glass, and ceramics; “ashes” including the residue of the combustion of solid fuels; “large wastes” including demolition and construction debris and trees; dead animals; “sewage treatment solids” including the material retained on sewage-treatment screens, settled solids, and biomass sludge; “industrial wastes” including chemicals, paints, and sand; “mining wastes” including slag heaps and coal refuse piles; and “agricultural wastes” including farm animal manure and crop residues.
Modern management of waste disposal began in the late 1800's and by the 1890's more than half of America's cities utilized some system of collection and disposal of refuse. Such refuse often included ashes, food and dry rubbish, which each had a specific secondary use. Food scraps were fed to animals on the farms, the ashes filled potholes in roads and “unhealthy” swamps, and the dry rubbish was sorted for valuables. Rags, paper and the like, made more paper, and metals went back into production as reusable goods. The secondary use of much early refuse made such disposal systems as modem day landfills unnecessary.
By the 1930's food scraps, rags and paper were mixed together and carted to an incinerator. Incineration was cheaper and easier than sorting the refuse for secondary use because the mixture of materials could be collected at one time and burned together, and incineration continues to be used today.
Incinerators of conventional design burn refuse on moving grates in refractory-lined chambers. The combustible gases and the solids they carry are burned in secondary chambers. In addition to heat, the products of incineration include the normal primary products of combustion including carbon dioxide and water, as well as oxides of sulfur and nitrogen and other gaseous pollutants. The nongaseous products are fly ash and unburned solid residue.
In the 1940's, sanitary landfills proliferated. A sanitary landfill is generally considered the cheapest satisfactory means of waste disposal, but only if suitable land is within economic range of the source of the wastes. Typically, collection and transportation costs account for seventy-five percent of the total cost of solid waste management. In modern landfills, refuse is spread in thin layers, each of which is compacted by heavy industrial equipment such as bulldozers before the next layer is spread. When about 3 meters of refuse has been laid down, it is covered by a thin layer of clean earth which also is compacted.
In any event, by the 1950's, with the explosion of consumer products focusing on disposability, the amount of refuse generated increased dramatically. In fact, some reports suggest that by the 1970's five pounds of garbage per capita were discarded daily as compared to 2.7 pounds in the 1920's. In the 1980's, the public began to appreciate that congested landfills were polluting drinking water. At this time, recycling and composting began a resurgence and, today, they are at the forefront of community living.
Recycling and composting is recognized as an efficient way to handle organic solid waste and to reintroduce nutrients into nutrient depleted soil. In addition, recycling has transformed discarded materials, such as cellulose, wood, grass, leaves, cardboard, pallets, tree limbs, etc., plastics (polystyrene, polyethylene, polypropylene, PVC, etc.), glass, and ceramics into reusable materials.
Although the benefits of recycling are recognized, by far the most common method of disposing of solid wastes in the United States is the deposition of such wastes on land or in “landfills,” which may account for more than ninety percent of the nation's municipal refuse. Incineration accounts for most of the remainder, whereas composting of solid wastes accounts for only an insignificant amount.
With regard to landfills, although pollution of surface and groundwater is believed to be minimized by taking such precautions as: lining and contouring the fill; compacting and planting the cover; selecting proper soil; diverting upland drainage; and placing wastes in sites not subject to flooding or high groundwater levels, such pollution remains a concern. Gases are generated in landfills through anaerobic decomposition of organic solid waste. If a significant amount of methane is present, it may be explosive; therefore, proper venting and burning of the methane gases are often necessary to eliminate or alleviate these dangerous conditions.
With regard to incineration, the process introduces harmful by-products and pollutants into the atmosphere and incineration methods are known to destroy the useful hemicellulose component of woody cellulose materials contained in solid waste (see the explanation set forth below).
Because landfill and incineration methods of disposal are known to pose significant environmental problems and concerns for the municipality, government, private industry, and individuals, recycling has become an attractive alternative. The treatment and handling of solid waste for reuse is particularly attractive. Such treatment and handling of solid waste is referred to herein as “resource recovery.”
Hammer mills incorporate rotating drums with free-floating hammers. They are designed to spin at a relatively high speed, such that material placed in front of the rotating drum is impacted by the hammers. Thus, hammer mills do not cut, shred or tear the material, but rely on impact forces to pulverize the material.
Grinders also incorporate rotating drums; however, grinder drums generally have a flat abrasive surface or include integral cutters, such that material placed in contact with the rotating drum is cut, torn, and shredded.
Shredders typically incorporate a pair of rotatable parallel shafts, having spaced apart cutters, which pull the material downward between the parallel shafts, causing the material to be shredded. In an overload condition, the rotation of the shafts may be momentarily reversed before resuming the shredding rotations, however, when a typical shredder becomes clogged with debris, it must be shut down for a lengthy de-clogging process.
Pressure vessel apparatuses, such as hydrolyzers, may be used for processing organic material, for example, animal carcasses or parts thereof, including organic wastes generated during meat and poultry production for human consumption. Such processing may be termed “metamorphic,” in that a change of physical form, structure, or substance to the components of the waste is effected. Known hydrolyzer apparatuses have various shortcomings. For example, these conventional vessels are prone to repeated and continuous clogging when trying to process certain waste material and thus require repeated down time intervals and disassembly to empty the interior of the vessel.
Turning now, from the machines, to the methods of resource recovery systems, certain resource recovery methods can be considered thermal processes, generally, but more specifically, combustion processes or pyrolysis processes. Pyrolysis, also called destructive distillation, is the process of chemically decomposing solid wastes by the introduction of heat in an oxygen-reduced atmosphere. This results in a gas stream containing primarily hydrogen, methane, carbon monoxide, carbon dioxide, and various other gases and inert ash, depending on the organic characteristics of the material being pyrolyzed.
Another approach to the treatment of waste is known as a “wet pulping process.” In a wet pulping process the incoming refuse is mixed with water and ground into a slurry in an apparatus referred to as a wet pulper—a machine that is similar to a large kitchen disposal unit. Large pieces of metal and other non-pulpable materials are separated by a magnetic separator, and the residue is used as landfill. The slurry from the pulper goes into a centrifugal device called a liquid cyclone, which separates heavier non-combustibles such as glass, metals, and ceramics. The heavy fraction goes to a glass and metal recovery system; the light fraction goes to a paper and fiber recovery system. Combustible residues are mixed with sewage sludge, mechanically dewatered, and incinerated. Noncombustible residues are used as landfill.
These aforementioned processes are among those which have been used in an attempt to transform solid waste into a more manageable form, however, the resulting end-product of these processes is not always useful.
The useful components of solid waste and. the problems associated with extracting such components using known resource recovery systems will now be discussed. “Woody” cellulose materials, found in most solid waste, includes cellulose, a carbohydrate of unknown molecular structure, but which may be represented by the empirical formula (C6H10O5); lignin, an organic substance closely allied to cellulose and forming the essential part of woody fibers; and hemicellulose, which serves as the binding agent for the constituent elements of the wood-like cellulose molecule and is useful in applications such as papermaking.
The hemicellulose is composed of two general classes of substances: (1) those collectively called xylons whose molecules are formed by polymerization of certain forms of pentose sugars; and (2) glucomannans, whose molecules are formed by polymerization of certain forms of hexose sugars, primarily glucose and mannose. These substances cannot be readily disassociated from cellulose-containing material without being destroyed. For example, combustion and pyrolysis processes are known to destroy the hemicellulose and prevent its use as a bonding agent to form other molecules of cellulose. In this regard, many techniques used in resource recovery systems, including, oxidation, hydrogenation, alkaline hydrolysis, pyrolysis and use of powerful solvents, have limited utility because of their harsh and destructive nature.
To summarize, the existing waste disposal systems have a variety of problems. Use of landfills and incinerators ignore the useful components of solid waste and pose significant environmental problems
Existing apparatuses of resource recovery systems are inefficient in that they must be shut down for significant periods of time when becoming clogged with debris.
Additionally, existing methods of resource recovery systems incorporate harsh techniques, which not only destroy useful components of solid waste, such as hemicellulose, but also, in the case of certain solvents and oxidants, pose environmental concerns.
Furthermore, in all known methods of resource recovery systems, the resultant product may include microbes or microorganisms that require further consideration prior to disposal. In such cases the resultant products are believed to remain waste materials not suitable for use or transformation into useful articles.
Accordingly, there remains a need in the art for apparatuses and methods of resource recovery which satisfactorily addresses the problems set forth above.